QSC RMX Series Technical & Service Manual Download Page 11

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RMX Series Technical Service Manual (2RU models)

TD-000098-00 rev. C

the arrangement of the output transistors might look somewhat
backwards: PNP transistors connected to the positive voltage rail,
and NPN on the negative rail. Not only that, but the transistors
attached to the positive rail are for pulling the output voltage
negative, and the ones on the negative rail swing the output
positive. The collectors all connect to ground, which allows them
to be mounted directly to the heat sink—metal-to-metal, without
insulators in between—for the best possible transfer of heat away
from the transistors. The emitters of the PNP and NPN transistors
are coupled through resistors to the positive and negative supply
rails, respectively, forming banks of common-emitter circuits
driving the supply rails. Consequently, the devices drive the rails
with the audio signal, which rides atop the DC. The output to the
speaker load is taken from the point between the positive and
negative reservoir capacitors; this is also where the negative
feedback is taken from. The nature of this arrangement, with audio
signal riding on the supply rails, is why the power supply has no
ground reference.

Another unusual characteristic of the grounded-collector output
section is that the signal at the output to the speaker is actually
opposite in polarity to the signal at the op amp output. This is why
the negative feedback resistor, R122, connects to the op amp’s
non-inverting input instead of the inverting input.

The output point of the circuit couples to the output connector
through an RLC network (R160, R161, R162, L100, and C124) that
serves as a high-frequency snubber and also helps keep the amp
circuitry stable when driving capacitive loads.

Clip detection

The output of the op amp also drives a group of four diodes (D102,
D103, D105, and D106) arranged as a full-wave rectifier. Normally,
the op amp’s output signal level is about 1 volt or less, which is all
it takes to drive the driver transistors.

But because this point is within the overall feedback loop, when
clipping occurs, the op amp approaches full open-loop gain and
puts out a much higher signal voltage to try to make the output
signal track the input. The four diodes rectify the voltage to drive
the clip indicator LED, LD100. The current exiting the full-wave
rectifier passes to ground through R127 and also drives the base of
transistor Q100 through R115. If the clip limiter is switched on,
Q100’s emitter is grounded, and when the voltage across R127
goes sufficiently negative to forward-bias Q100, which sends
current through R111 and R103 into the amplifier bias input of the
operational transconductance amplifier (OTA), U10:1. The OTA is in
the negative feedback loop of U101:1, and increasing its
transconductance essentially reduces the impedance of the
feedback loop, which reduces the gain of the op amp stage. This
reduces the signal level until the amount of clipping is minimal.
When the clipping stops, Q100 is no longer forward-biased, and
the gain returns to normal.

The next active device is another 5532 op amp, U101:2. Its output
drives the driver transistors, which in turn drive the output transis-
tors. The output section has a Class AB+B configuration; the
drivers (a complementary pair, Q105 and Q106, comprising an NPN
MJE15032 and a PNP MJE 15033) are class AB. A series network
of two diodes and a 100-ohm trimpot provide the small amount of
forward bias on the transistor pair to keep crossover distortion
minimal. In parallel with the trimpot is a 50-ohm thermistor with a
negative temperature coefficient; as the circuitry warms, its
resistance decreases. This reduces V

on both Q105 and Q106,

decreasing the bias current to reduce the threat of thermal runaway.
The base of each driver transistor is tied to ground through a diode
and a 2.2 k

trimpot in series; these set the current limiting

threshold for their respective signal polarities.

The collector of each driver transistor directly drives the bases of
its output transistors, which are the main power-handling signal
devices. If you’re not familiar with the grounded-collector scheme,

100

^R_0805

R109

100

^R_0805

10.0K

^R_0805

R103

U10:1

LM13600M

R108

7.50K